The present invention relates to intracranial pressure guages and, more particularly to high precision gauges which are capable of measuring changes in intracranial pressure, not by physical connection with a wire or tube but by inductive coupling of a sensor implanted under the scalp with an external measuring device.
With conventional intracranial pressure gauges in use for chronic continuous measurment of changes in intracranial pressure of patients, the intracranial pressure has been directly measured by withdrawing signals from a sensor implanted under the scalp by means of a physical connecting material such as a wire or a tube. However, such gauges involve the risk of contamination from the outside because of the connecting material penetrating the scalp.
Some devices have been devised for the elimination of this disadvantage, for example, by connecting the sensor implanted under the scalp with an external measuring device such as by radio communication, and not by utilizing a physical connection. However, these devices have active circuits built in and accordingly are complex in structure, large in size, and tend to be involved in drift, etc. In addition, they employ component elements with such a low heat resistance that their sterilization cannot be achieved without resorting to incomplete and inefficient means such as gas application in place of heat application. Also, they cannot endure a long-term use since their built-in power source has a limited service life.
Conventional intracranial pressure gauges are, in general, implanted under the scalp separately from a shunt tube path. The shunt tube path is a separate catheter for discharging brain fluid with increased intracranial pressure to a point outside the brain cavity. This arrangement necessitates two catheters to be inserted into the ventricle. They require an additional tube path for the air release and clearing of the route within the sensor. Thus, they have the disadvantage that the size and volume of the device to be implanted under the scalp cannot be reduced very much.
Moreover, if air is sealed in the pressure-sensitive section of the sensor, the air pressure is caused to change with the changing temperature of the human body, which adversely affects the precise sensing of the intracranial pressure. It is possible to correct the intracranial pressure to adjust for the change in body temperature, but the correction for temperature is not practically available since the body temperature is more or less dependent on the location of measurement, and the change in intracranial pressure is too small for the application of the temperature correction technique.
Additionally, the intracranial pressure gauge requires high sensitivity up to atmospheric pressure in order to be capable of measuring an extremely small change in intracranial pressure relative to the atmospheric pressure.
The present invention has applied the principle of resonance grid dip to the solution of the above problems. According to this principle, when an induction-powered resonance circuit consisting of a coil and a condenser is implanted under the scalp, an external inductive coupling allows the resonance point for the implanted resonance circuit to be detected. An extensive investigation had led to the discovery that, when the induction-powered resonance circuit is made variable with respect to the inductance L (simply stated as L hereinafter) of its coil, to the capacitance C (simply stated as C hereinafter) of its condenser, or to both L and C and the relation has been established between the resonance frequency for L and C and the change in intracranial pressure, the above-mentioned inductive coupling may externally furnish indirect measurments of the intracranial pressure. The intended purpose of the present invention is achieved by inserting the sensor of the intracranial pressure gauge in the shunt tube path, thereby eliminating the dual tube paths, by evacuating the pressure-sensitive section of the sensor, thereby eliminating the bad effect of the included gas, if any, with temperature change, and by utilizing the beat phenomenon for synthesized wave for detecting fine changes in intracranial pressure, thereby facilitating the measurement.
Accordingly, one of the objects of the present invention is to apply the concept of constituting a passive circuit without any built-in power source for making available a novel intracranial pressure gauge which consists of three components: (1) a powerless resonance circuit composed of a high-precision coil and condenser which is connected to an external measuring device by other means than physical connecting elements, resulting in no risk of contamination from the outside environment, a long-term availability, the applicability of thermal sterilization of the sensor prior to implantation, and an extremely reduced volume of the sensor; (2) a sensor equipped with a pressure-sensitive section capable, when implanted under the scalp, of changing either the L of the coil or the C of the condenser in response to the change in intracranial pressure; and (3) a grid dip meter capable of externally measuring the change in the resonance frequency of the sensor.
Another object of the present invention is to provide another type of intracranial pressure gauge which consists of three components: (1) a powerless resonance circuit composed of a coil and a condenser; (2) a sensor equipped with a pressure-sensitive section capable, when implanted under the scalp, of allowing the provided core to move within the coil as the change in intracranial pressure causes the bellows to change its length, thereby changing the L of the coil; and (3) a grid dip meter capable of externally measuring the change in the resonance frequency of the sensor.
Still another object of the present invention is to offer another type of intracranial pressure gauge which consists of three components: (1) a powerless resonance circuit, composed of a coil and a condenser, with increased sensitivity by making the magnetic path of the core continuous; (2) a sensor equipped with a pressure-sensitive section capable, when implanted under the scalp, of allowing the cylinder section of the provided core, in the form of a hat, equipped with a flange at the bottom side (brain side) of the cylinder section, to move within the coil as the change in intracranial pressure causes the provided metallic bellows to change its length, thereby changing its L; and (3) a grid dip meter capable of externally measuring the change in the resonance frequency of the sensor.
Still another object of the present invention is to offer another type of intracranial pressure gauge which consists of three components: (1) a powerless resonance circuit composed of a coil and a condenser, for which two shunt tube paths which communicate with each other make it possible to reduce the volume of the device to be implanted under the scalp and to release air and clear contaminations from the sensor; (2) a sensor equipped with a pressure-sensitive section capable, when implanted under the scalp, of changing either the L of the coil or the C of the condenser in response to the change in intracranial pressure and with a shunt tube path for discharging brain fluid from the ventricle, having the pressure-sensitive section inserted; and (3) a grid dip meter capable of externally measuring the change in the resonance frequency of the sensor. A still further object of the present invention is to offer another type of intracranial pressure gauge which consists of three components: (1) a powerless resonance circuit composed of a coil and a condenser; (2) a sensor equipped with a pressure-sensitive section capable, when implanted under the scalp, of changing either the L of the coil or the C of the condenser in response to the change in intracranial pressure conveyed via the meninges; and (3) a grid dip meter capable of externally measuring the change in the resonance frequency of the sensor.
Yet another object of the present inventon is to offer another type of intracranial pressure gauge which consists of four components: (1) a powerless resonance circuit composed of a coil and a condenser, which is capable of precise and easy measurement of fine changes in intracranial pressure; (2) a sensor equipped with a pressure-sensitive section capable, when implanted under the scalp, of changing the L of the coil in response to the change in intracranial pressure; (3) a grid dip meter capable of externally measuring the change in the resonance frequency of the sensor; and (4) a device capable of synthesizing the resonance frequency signal of the grid dip meter and the standard frequency signal.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.